The present invention pertains to low density, flexible microcellular elastomers suitable, inter alia, for the preparation of molded shoe soles, inners, and midsoles.
Cushioned soles for footware, particularly athletic footware are generally prepared from EVA (ethylenevinylacetate) microcellular foams. The processing of such foams is not straightforward, and the foams themselves do not have optimal properties. However, such foams continue to be used due to their availability in the very low density range, i.e., 0.1 g/cm3 to 0.35 g/cm3.
Polyurethane polymers generally exhibit physical properties which are superior to EVA polymers. However, numerous difficulties arise when attempts are made to mold polyurethane microcellular foams at low densities. Due to the hardness required for the end use, considerable amounts of low molecular weight chain extenders are required. In prior microcellular foams which are water-blown, the urea short segments which are created cause the formulations to have poor processability, resulting in both shrinkage and splits in the parts. The physical properties are also comprised, particularly when the superior properties of otherwise similar noncellular polymers are considered. These problems have prevented use of low density ( less than 0.75 g/cm3) polyurethane microcellular foams, and more particularly, very low density ( less than 0.35 g/cm3) polyurethane microcellular foams.
It has been surprisingly discovered that low density, and particularly very low density, polyurethane flexible microcellular elastomers may be prepared if the major portion of the water blowing agent is replaced by dissolved CO2. Polyurethanes produced in this manner from ultra-low unsaturation polyols exhibit mechanical properties, including relatively high hardness at low density, which makes them eminently suitable for use in shoe sole components. Yet more surprising, polyester polyol-based microcellular, flexible polyurethane elastomers may be easily produced, even though their production as water-blown microcellular foams was exceptionally difficult.